The present invention relates generally to wireless communication systems, in which are disposed multiple base stations, each of which broadcasts a distinguishing pilot signal. More particularly, the present invention relates to a novel and improved technique for the maintenance of an active pilot set by mobile stations.
Handoff is the process of transferring a wireless call from one transceiver, a source transceiver, to a second transceiver, a target transceiver. There exist a number of different methods for conducting a handoff in a cellular and other wireless communication systems. For example, many systems utilize a hard handoff between transceivers (or base stations). A hard handoff is a handoff in which the call connection with the first transceiver or base station is broken prior to establishing the call connection with the second transceiver or base station. While hard handoffs may be performed quickly, the call quality is still affected during a hard handoff; and connections with the second transceiver or base station may be missed altogether, resulting in calls being dropped.
To minimize the effect of these problems, the idea of soft and softer handoffs were developed. A soft handoff is a handoff between one site and another. A softer handoff is a handoff between one sector and another sector, both located at the same site. The basic concept of soft/softer handoff is that the connection is established with the second transceiver before the connection with the first transceiver is broken. One type of system that utilizes soft handoffs is a code division multiple access (CDMA) system, although soft handoff schemes may be generally employed in any radiotelephone system in which the mobile station or device has the capability of monitoring the strength of signals transmitted by a plurality of base stations in parallel with the communication of an active call.
A CDMA system utilizes the concept of pilot channels. A pilot channel is an unmodulated, direct-sequence spread spectrum signal transmitted continuously by each CDMA base station. The pilot channel allows a mobile station to acquire the timing of the forward CDMA link (i.e. xe2x80x94handoff leg), provides a phase reference for coherent demodulation, and provides a means for signal strength comparisons between base stations for determining when to handoff. These pilot channels or pilots are generally assigned into one of a number of pilot sets by the mobile station, according to the status of these pilots with respect to the mobile terminal. A common classification of pilot sets is to divide such sets into the Active Set, the Candidate Set, the Neighbor Set and the Remaining Set. The Active Set is a set of those pilots that are selected for demodulation and combination by the mobile station; that is, Active Pilots participate in the call. The Candidate Set includes a list of those pilots that have been received by the mobile station with sufficient strength to be successfully demodulated, but have not been placed in the Active Set. Candidate Set pilots are thought of as being xe2x80x9cnext-in-linexe2x80x9d for inclusion in the Active Set. Neighbor Set pilots are those pilots that are included in neither the Active Set nor the Candidate Set, yet are still considered likely future candidates for handoff. Normally, the Neighbor Set includes those pilots associated with CDMA channels that cover geographical areas near the mobile station. Remainder Set pilots are those CDMA network pilots of the current carrier frequency assignment that are not included in one of the previously described sets.
In a typical implementation of a mobile station""s handoff management algorithm, handoff legs (forward links) are added/dropped to/from the Active Set according to some criteria based on ADD/DROP thresholds. These thresholds are either pre-specified by the designer or left to the base station to vary in some dynamic, adaptive manner that suits system requirements. In each case, the mobile station""s ADD/DROP threshold setting algorithms and schemes are based on a subset of measurements collected for Active Set pilots only. Unfortunately, use of a linear combination of the Active Set pilot parameters in the threshold setting procedure may yield inappropriate or less than maximally efficient handoff decisions. For example, increasing/decreasing the ADD threshold value based on the simple sum of the Active Set pilot strengths may exclude useful legs from being added to the Active Set, or in the alternative, may result in inclusion of legs that offer no significant benefit. Similarly, increasing/decreasing the DROP threshold based on the simple sum of the Active Set pilot strengths may result in dropping otherwise beneficial legs from the Active Set, or in the alternative, preserving legs that offer no significant benefit. Generally, improperly adding or dropping legs may lead to the addition of unnecessary legs which only increase processing complexity; or may lead to an insufficient number of legs which can jeopardize the integrity of the call.
Additionally, improperly adding or dropping legs may cause xe2x80x9cping-ponging,xe2x80x9d a term used to describe an unwanted, repetitious process in which effective legs are dropped and then re-added shortly afterwards, or in which ineffective legs are added and then dropped shortly afterwards.
Handoff efficiency is increased and ping-ponging reduced in a wireless communication system, in which a mobile station is operable to monitor the signal strengths of a plurality of pilot signals transmitted from a plurality of base stations, by a method and device in which the mobile station determines the membership of Active Set pilots by generating a quality metric derived as a function of (i) detected pilot signal strengths corresponding to the Active Set of pilots, and (ii) detected pilot signal strengths corresponding to a non-Active Set of pilots, determining a threshold value, the threshold value determined as a function of the quality metric, and assigning the pilot to, or removing the pilot from, the Active Set of pilots if the signal strength corresponding to the pilot exceeds the threshold value.